Adds an event spy for the xml file used by gtk following the example of the KRecentDocument EventSpy.

It allows to populate recently accessed files from apps using Gtk, including inkscape, gimp...

CCBUG: 311218

Test Plan

Compile
killall -9 kactivitymanage
run your localy built version of kactivitymanagerd, for instance:
~/kde/usr/lib/x86_64-linux-gnu/libexec/kactivitymanagerd
Run a gtk app and open a file
The file access can be seen in kickoff/kicker history

Can you check out the KBookmarks framework which has classes for processing xbel files

Thanks for pointing it out.
I just did, and It does not seem to cover my use case here :
It is quite heavily oriented towards its main use : application bookmark.
For instance to get the attributes of the bookmarks elements I would need to use the internalElement() method to get a raw QDomElement which kind of defeat the purpose of using a library.
I would also need to parse myself the <application> elements using internalElement as well.
While user-places.xbel references https://www.freedesktop.org/wiki/Specifications/desktop-bookmark-spec/ as its specification, it does not abide by it.